1. Field of the Invention
The present invention relates to a process for preparing mesoporous crystalline material, in particular to a novel process for preparing crystalline material having mesoporous structure by dissolving silica in hydrogen fluoride solution and reacting it on the condition of atmosphere and room temperature for a short reaction time.
As the result, the present invention provides a process to use all kinds of silica and to synthesize mesoporous crystalline material for a short reaction time.
2. Description of the Prior Art
Porous inorganic materials have found great utility as catalysts or separation media for industrial application. The opening of their microstructure allows molecules to access to the relatively large surface areas of these materials that enhance their catalytic and sorptive activity when reaction occurs.
The porous materials in use today can be sorted into the broad categories using the details of their microstructure as a basis for classification. These categories are the amorphous support, crystalline molecular sieves and modified layered structures to intercalate small fragments like metal oxide.
The microstructure of these materials shows the detailed differences and due to said differences, their specific selectivity is manifested in the catalytic and sorptive behavior. Further, the above differences are ascertained in terms of their surface area, the size of pores and the variability in those size and X-ray diffraction patterns. The microstructure appearance of the materials is also shown through transmission electron microscopy.
Generally since amorphous silica or alumina support forms the pore resulting from voids between particles, they don't have uniform pore size and show the pore size of wide range of 13.about.200 .ANG..
In contrast to these structurely ill-defined materials, the solids of regular crystallinity like zeolite have very narrow pore size distribution. The majority of natural or synthetic zeolites have the pore of limited size and mesoporous zeolites having uniform distribution of ZSM series were synthesized in an effort to enlarge the pore size.
In addition, for using the layered silica and metal phosphate compounds including natural or synthetic clays, the synthesis of fine porous materials was attempted. The pore size distribution is regular compared to amorphous support but irregular compared to crystalline support like zeolite.
Recently, novel mesoporous crystalline materials were successfully synthesized by Mobil Co. Their pore size can be controlled uniformly in the regime of 20.about.100 .ANG.. It was named as MCM-41 and U.S. Pat. Nos. 5,108,725; 5,102,643; 5,098,684 and 5,057,296 describe synthesizing process, reactants for synthesis, composition, physical and structural property.
In the above mentioned prior arts, they demonstrated a synthesizing process for preparing mesoporous crystalline materials by using tetraammonium hydroxides like tetramethyl ammoniumsilicate solution as well as surfactant in an autoclave at a temperature of 100.about.150.degree. C. for 2.about.7 days. The reaction principle of this process is attracting silica or silica allumina crystal on the micelle colloid surface having charge, which is formed by dissolution of surfactant molecules in water. Ignition may form a bond and crosslink with the bridge of oxygen between Al and Si in the molecular dimension. To satisfy said circumstance, a large amount of tetraammonium hydroxides is used and reaction condition is controlled at high temperature and high pressure for 2.about.7 days.
Materials prepared by said process have not only uniform pore size but also remarkable thermal property and large surface area of 500.about.1000 m.sup.2 /g.
The constituents of framework consist of SiO.sub.2 and SiO.sub.2 /Al.sub.2 O.sub.3. In view of remarkable physical properties, these new materials will replace the conventional zeolites and be widely used in the future.
Silica is hydrolyzed with the form of hydroxide anion in strong aqueous alkali solution and dissolved. Formed silicon hydroxide anions at this very moment make a wall of pores by forming network by means of silicon bonding because polymerization occurs on the micelle colloid surface having positive charge if miscelle colloid of cationic surfactant exits.
Generally the solubility of silica largely depends on the kind of alkali, particle size of silica, density, temperature and pressure. Alkali-metal hydroxides are largely used in synthesizing of crystalline silicate but its use is not preferred because a large amount of impurities such as salts (Na.sub.2 SO.sub.4, NaCl) can be deposited during the neutralization process when used in synthesis of MCM-41.
If bulk density of silica is sufficiently low and particle size is sufficiently small, the large quantity of silica may be dissolved comparatively in the organic base such as tetramethyl ammonium hydroxides. But these procedures have disadvantages that the kind of usable silica is limited and particular conditions of high temperature, high pressure and long of time are required.
Moreover, tetramethyl amonium hydroxides is expensive and decomposes to amine and alcohol at high temperature and it is technically and economically difficult to manufacture and operate reaction apparatus of high temperature and pressure.
In said process, a large amount of tetra ammonium hydroxides of high price are used and it is not economical. And there are additional disadvantages in the process in that the reaction ought to be carried out at high temperature and high pressure for a long time as the well as disadvantage that only raw material of silica, for example highsil, ultrasil or fumed silica, which are voluminous and easy to fly is used to facilitate dissolution.
Accordingly, in order to solve the above mentioned problems, inventors have made an effort to manufacture more economically and in a short time the crystalline materials having conventional mesoporous structure named MCM-41 by using all kinds of silica. And finally, we have developed a novel process for preparing mesoporous crystalline materials by using hydrogen fluoride solution with all kinds of silica for a few hours at a temperature range of 25.about.80.degree. C.